1. Field of the Invention
This invention relates to a process for catalytically cracking hydrocarbons with the simultaneous production of a low BTU fuel gas. More particularly, this invention is a process for catalytically cracking hydrocarbon feeds over solid acid catalysts comprising a catalytic metal oxide component wherein said metal is selected from the group consisting essentially of (a) tungsten, niobium and mixtures thereof and (b) mixtures of (a) with tantalum, hafnium, chromium, titanium, zirconium and mixtures thereof, said metal oxide component being supported on a silica-containing inorganic refractory metal oxide support, the silica content of which is less than 50 wt.% calculated as SiO.sub.2 and simultaneously producing a low BTU value fuel gas via regeneration of the deactivated and coked catalyst resulting from cracking the feed.
2. Background of the Disclosure
A typical conventional process for the conversion of heavy hydrocarbon feeds (resids, coal liquids) is fluid coking. Ordinarily the process operates with two fluidized beds, a reactor and a burner. The hydrocarbon feed is injected into the reactor where it is thermally cracked to form vapor phase products and coke. Alternatively, heavy feeds can be catalytically cracked. This process also functions with multiple fluidized beds with catalyst recirculating between a reactor and a regenerator. The feed to be cracked is injected, with the hot, regenerated catalyst into the reactor where the cracking reaction occurs. The hydrocarbon products and the catalyst are separated by steam stripping; the products are sent to a fractionator and the catalyst is transferred to the regenerator. In the regenerator deposited coke is removed by burning in air and the regenerated catalyst is than returned to the reactor. Depending on the catalyst used, regenerator temperatures are between 1200.degree. and 1400.degree. F. (650.degree.-760.degree. C.). There are currently three catalysts commonly in use for catalytic cracking: silica-alumina, zeolite silica-alumina mixtures and silica-magnesia.
To avoid serious catalyst deactivation and excessive coke make, petroleum feeds sent to cat cracking processes are normally restricted to the vacuum gas oil fraction boiling below 1050.degree. F. This is directly related to the large amount of metals in the 1050.degree. F..sup.+ material that contaminates the catalyst and the large amount of Conradson Carbon coke forming precursors in the 1050.degree. F..sup.+ material. Techniques are available to mitigate the effects of deposited metals, e.g., antimony addition, however, the high content of coke precursors in 1050.degree. F..sup.+ materials still make direct processing in cat cracking problematic. Presently, catalysts which have become deactivated due to coke deposition are regenerated by burning the coke. If unhydrotreated 1050.degree. F..sup.+ was processed in large quantities in cat cracking the amount of coke produced would be greater than the amount that could be burned in the regenerator to sustain the unit in heat balance. The heat produced by burning off the excess coke would likely have to be used in the production of relatively low value steam. This will be accomplished by incorporation of steam boiler tubes in the regenerator bed. Ideally, however, the coke should be converted into valuable products since the coke for a 1050.degree. F..sup.+ material represents about 25% of the total material fed into the cat cracking unit; about three quarters of which is in excess over the amount that would be burned to sustain the unit in heat balance. In order to convert the coke into commercially valuable products it must be gasified, that is, reacted with an oxygen-containing and/or steam-containing gas at temperatures of 1600.degree.-1800.degree. F. (870.degree.-980.degree. C.).
One of the main drawbacks in attempting to utilize this coke gasification in a typical cat cracking process is the extreme sensitivity of the prior art catalysts to high temperatures in the presence of steam. While the coke may indeed be gasified the catalyst is irreversibly deactivated, thereby resulting in a very high debit in operations. Consequently, to avoid destroying the catalyst, the coke is merely burned at low temperatures (&lt;1400.degree. F.) and when this quantity of coke is greater than the amount needed to sustain heat balance, the excess is used to produce steam for the refinery, an equally pernicious result considering that coal could be most readily used in large stationary combustors such as steam boilers. In order to achieve maximum material efficiency and practice cat cracking of substantial quantities of 1050.degree. F..sup.+ streams, it will be necessary to simultaneously practice both coke gasification and catalyst regeneration. To do this, an exceptionally long lived, active, stable catalyst will be needed.
With the incorporation of gasification, the coke gas produced is mainly a mixture of CO, CO.sub.2, H.sub.2, H.sub.2 O, H.sub.2 S and N.sub.2 if air is employed. The H.sub.2 S can be removed by technology such as the Stretford process, thereby a clean H.sub.2 -CO containing gas might be produced for a wide range of uses -e.g. process furnaces, fuel gas, H.sub.2 manufacture, etc. Gasification is therefore a more efficient utilization of this potential coke energy contained in each barrel of feed. For the case where air is used as the gasification medium, the coke gas produced, about 10.sup.4 SCF/bbl, would have an energy content of about 100 BTU/SCF.
Cracking of petroleum fractions over acid catalysts represent the most widely used means of molecular weight reduction in refining processes. In order to achieve hydrocarbon cracking in the presence of steam, the catalyst used in the process must retain a high level of acid cracking activity in the presence of steam.
Most of the conventional acid catalysts used in catalytic cracking processes are known to be unsuited for use in the presence of steam at temperatures greater than about 1400.degree. F. (760.degree. C.). The effects of water are considered to be those of a structural poison and result in a weakening of the acid strength of the acid sites. Stability in high steam environments is desirable however, since one way of removing the coke which is inevitably deposited on the catalyst is to steam gasify the coke on the catalyst. The burning off of the coke necessarily requires the presence of steam.
U.S. Pat. Nos. 4,269,737, 4,233,139 and 4,244,811, the disclosures of which are incorporated herein by reference, disclose solid acid catalysts comprising certain supported transition metal oxides, their preparation and use as acid cracking catalysts. The supports are silica-free refractory metal oxides which are not in themselves acid cracking catalysts, but which when combined with the transition metal oxide component result in acid cracking catalysts. These catalysts are taught as having enhanced activity and selectivity towards forming liquid products compared to conventional acid catalysts. They also exhibit remarkable resistance to coke make and have unusual steam stability. In fact, in some cases it is preferred to pre-steam these catalysts prior to use in order to stabilize the surface thereof. Although steaming these catalysts initially results in a decrease of both surface area and strong acid sites, the steaming itself serves to stabilize the remaining surface area and acidity which is not substantially adversely affected by subsequent steaming. The catalysts described in these patents exhibit primarily Lewis acidity.
U.S. Pat. No. 2,849,383 discloses silica-alumina cracking catalysts which may contain tungsten oxide as a component thereof, which catalysts are steamed at a temperature of from about 600.degree.-800.degree. C. prior to use. However, the disclosures of this patent are very specific in teaching that the silica content of the catalyst support should be at least above about 50% silica calculated as SiO.sub.2 and preferably between 50 and 90 wt.% silica, with the balance being alumina.